Just an update for anyone that finds this in the future:
I moved forward with the OMV VM plan. I installed proxmox and created a VM for OMV and passed a 9361-8i raid controller through to the VM. One port on the 9361 is connected to a SAS expander card, AEC-82885T, which is then connected to nine, at the moment, Exos 20TB SATA drives. I've got one set as a hot spare and seven others in a 5+2 RAID 6. I plan to add the last drive after background initialization of the array is finished. BGI is taking a looooong time, as expected. But I have started using the array already in OMV and everything is going well.
As for the OMV config, I have installed the LVM plugin in OMV and used the RAID 6 array as a PV in LVM. I created a VG from that and then LV. The LV is used as a BTRFS Single filesystem and I've created several shares. I've been adding data the last couple days and things are pretty fast despite BGI still taking place. All in all, I'm happy with the results so far.
I'll update here if anything noteworthy comes up. Thanks!